Present invention relates to a production technique for semiconductor device and particularly to a technique effectively applied to methods of effecting exposure dose and focus control in an exposure step and further effecting pressure and temperature control in an etching step with high accuracy.
For example, production of semiconductor devices is carried out by repeating on each layer a film formation step of forming a conductive film or insulating film on a silicon wafer and a lithography step of coating on the film a resist that is a photosensitive material, exposing and developing a circuit pattern on a reticle by an exposure system, and then forming the circuit pattern on the wafer by using the remaining resist as a mask to etch the film.
Here, as a background of the present invention, a technique examined as a premise for the present invention by the present inventors will be described with reference to FIGS. 21 to 26. FIG. 21 is a view for explaining an exposure step of a semiconductor; FIG. 22 is a view showing a structure of a scatterometry pattern measuring system of spectroscopic waveform system; FIG. 23 is a view showing a line and space pattern on a wafer; FIG. 24 is a view showing another structure of the scatterometry pattern measuring system of spectroscopic waveform system; FIG. 25 is a view showing a structure of a scatterometry pattern measuring system of angle-scanning type; and FIG. 26 is a view for explaining a shape measurement method by the scatterometry pattern measuring system.
First, in the lithography step, the exposure step in which circuit patterns are printed on a resist is explained using FIG. 21. The circuit patterns to be formed on a layer are drawn in a reticle 200. The reticle includes an area 201 in which a product circuit pattern is drawn and an area 202 in which a test circuit pattern on a periphery thereof is drawn. All the patterns are transferred via an exposure lens 204 onto the resist coated on a wafer 1 by exposure light 203. A transferred and exposed portion of the resist remains in a subsequent development step to serve as a mask (in the case of a negative type resist). In order to confirm whether the transferred circuit patterns have been completed with such exact dimensions as to be designed, a dimensional inspection is commonly conducted using a Scanning Electron Microscope (SEM).
The inspection is conducted, among the transferred circuit patterns 200t, either by directly measuring a transferred product pattern 201t or by measuring transferred test patterns 202t existing outside a chip region. Depending on the measured dimensional size, correction is generally carried out with exposure dose of the exposure system. Automation of this correction of exposure dose is disclosed in, for example, “Implementation of a Closed-loop CD and Overlay Controller for sub 0.25 μm Patterning”, SPIE Vol. 3332, 1998. pp 461-470. Meanwhile, except for exposure dose variation of the exposure system, focus deviation is recited as a cause of dimensional variations. A method of correcting not only for exposure dose but also for focus is disclosed in, for example, Japanese Patent Laid-Open Publication No. 2001-143982. This is a method of determining correction amounts for the exposure dose and focus directly from waveforms of SEM by relating a change in the waveforms of SEM to the exposure dose and focus deviations in advance. Further, as to inspection of the circuit patterns on the resist, it has found that not only dimensions but also height-directional shapes of the pattern, that is, a cross-sectional profile affects performance of the semiconductor device. Therefore, a scatterometry, i.e., a method of measuring optically a cross-sectional profile of the transferred circuit pattern is disclosed in, for example, “Specular Spectroscopic Scatterometry in DUV Lithography”, SPIE Vol. 3677, 1999, pp 159-168.
Here, a structure of a scatterometry pattern measuring system is explained with reference to FIG. 22. FIG. 22 shows a spectral type scatterometry pattern measuring system.
After light outputted from a white light source 21 is guided to a condenser lens 22 and polarized by a polarizing element 28, the light is radiated, as an incident light 23 with an angle, onto a wafer 100 that is a measurement object on a wafer stage 101. Reflected light 24 from the wafer 100 passes through a polarizing element 28a and is condensed to a grating 26 through a condenser lens 25. The reflected light 24 is spectrally divided by the grating 26, spectrally resolved by a spectroscope 27, and outputted as a spectral waveform 29a by a waveform processing unit 29. Further, the wafer 100 is mounted on a wafer stage 101, and waveform detection is possible at an arbitrary position. The polarizing elements 28 and 29 are capable of changing polarization directions in accordance with directions of the patterns, so that the waveform detection of all of TE polarization parallel to the pattern and TM polarization perpendicular thereto, or that of polarization at an intermediate angle (such as 30 and 45 degrees) can be carried out.
Here, the position to be radiated on the wafer 100 is, for example, a line and space area as shown in FIG. 23. This is because a sufficient quantity of light has to be obtained and uniform patterns are required. Therefore, a shape of being constituted by lines 206L and spaces 206S in FIG. 23 within an area of approximately 50 μm square or larger is required. Such patterns do not commonly exist in the product circuit, so that they are formed in the test pattern area 202t and outside the product circuit patterns.
In addition thereto, a structure of the optical system as shown in FIG. 24 is also known. Incident light 30 outputted from the white light source 21 is reflected by a half mirror 31, condensed by an objective lens 32, and radiated via the polarizing element 28 onto the wafer 100 that is a measurement object. Reflected light 33 from the pattern on the wafer 100 passes through the polarizing element 28 again, passes through the objective lens 32 and the half mirror 31, spectrally divided by the grating 26, and becomes spectral data by the spectroscope 27. Accordingly, a spectral waveform of the objective pattern can be obtained.
In the foregoing, the structure of the spectral type optical system has been shown, and an example of an angle-scanning type optical system is also disclosed. The example is explained with reference to FIG. 25. Light such as laser that is outputted from a single-wavelength light source 40 is radiated, as an incident light 42 with a specified angle of θ from an irradiation end 41, onto the wafer 100 that is a measurement object. Reflected light 43 from the pattern on the wafer 100 is detected by a light receiving element 44. The irradiation end 41 is arranged on an emission-angle changing unit 46 and the light receiving element 44 is arranged on a light-receiving angle changing unit 47, whereby the reflected light with an arbitrary angle can be detected. The light from the light receiving element 44 is transmitted to a detecting unit 45, whereby a reflection strength waveform 45a at a measuring point with respect to each angle is measured.
Next, a means for obtaining a shape of an object using a waveform detected by the above optical system is explained with reference to FIG. 26. An optical waveform obtained from a periodic pattern varies depending on a shape of a measurement pattern. Therefore, the optical waveforms with respect to various cross-sectional profiles are sought in advance by the wave optical simulation and are stored as a library. For example, the cross-sectional profile is modeled in accordance with pitch P, bottom line width W, film thickness H, and taper angle φ of a repetitive pattern cross-sectional shape 211, whereby the simulation is carried out. A simulation result 212 is stored in a library 213. Comparison of a measured waveform 210 with those stored in the library is carried out by a comparing means 214, and a cross-sectional profile having a waveform matched to the measured waveform is determined as a measurement value. The comparing means 214 uses only the library, carries out the optical simulation in real time without using the library, or takes a combination of both in accordance with required accuracy.
In comparison with the SEM in which there is a possibility that the line width is changed during radiation of electron beams due to a reaction of a photosensitizer, such a scatterometry is advantage in that the measurement is made by the light. Further, the scatterometry is capable of measurement in the atmosphere and it does not take time to carry out vacuuming unlike the SEM, whereby the high speed measurement can be made.
The scatterometry has a merit in comparison with the SEM in measuring the above-mentioned cross-sectional profile of the circuit pattern. However, since preliminary computation of a large number of waveforms is necessary, it is required to carry out the optical simulation at a high speed. Due to this, a computation method called “Rigorous Coupled Wave Analysis” disclosed in, for example, “Diffraction Analysis of Dielectric Surface-relief Gratings”, J. Opt. Soc. Am., Vol. 72, No. 10, 1982 is adopted. This is a method of seeking coefficients of series solution of a wave equation by: approximating a pattern section by a plurality of rectangular layers; remarking the respective rectangular layers as a diffraction grating with the same pitch and duty that continue endlessly; and matching boundaries thereof with each other. The above method carries out the waveform computation at the significantly high speed in comparison with a finite element method that is another solution of the wave equation.
A method of carrying out the focus control by relating the pattern cross-sectional shape and the focus deviation, which have been measured using such scatterometry is disclosed in, for example, “The Proceedings of the 64th Autumn Meeting of The Japan Society of Applied Physics”, 1p-R-2, P 641 (2003).
Meanwhile, in the above scatterometry, when the focus margin in the exposure step to be applied is small and detection accuracy of defocus is required, sensitivity to a change of the cross-sectional shape with respect to focus change becomes important. However, in the method by the above simulation, an influence on computation errors of cross-sectional shape owing to error between dimensions of the simulation and a practical device has not be ignored.
Further, the patterns of the measurement object in the scatterometry are required to have lines and spaces of the same pitch and duty within a range of several tens μm that serves as an illumination area, so that they are not always within an area on the smallest focus margin in practice. Thereby, the problem is such that the focus detection on the product circuit patterns cannot be made.